Security researchers say Microsoft customers should take immediate action to defend against the ongoing cyberattacks, and must assume they have already been compromised.

Microsoft is issuing an emergency fix to close off a vulnerability in Microsoft’s SharePoint software that hackers have exploited to carry out widespread attacks on businesses and at least some federal agencies.

The U.S. Cybersecurity and Infrastructure Security Agency (CISA), in an alert, said it's aware of active exploitation of CVE-2025-53770, which enables unauthenticated access to SharePoint systems and arbitrary code execution over the network.

The hackers behind the initial wave of attacks exploiting a zero-day in Microsoft SharePoint servers have so far primarily targeted government organizations, according to researchers and news reports.

Microsoft also has issued a patch for a related SharePoint vulnerability — CVE-2025-53771; Microsoft says there are no signs of active attacks on CVE-2025-53771, and that the patch is to provide more robust protections than the update for CVE-2025-49706.

M icrosoft has released two emergency patches to address zero-day vulnerabilities that have been found in SharePoint RCE. Actively exploited in attacks, the two flaws (tracked as CVE-2025-53770 and CVE-2025-53771) are both “ToolShell” attacks that compromise services and that build on flaws that were fixed as part of July’s Patch Tuesday updates.

A cyber-espionage campaign centred on vulnerable Microsoft software now involves the deployment of ransomware.

More information has emerged on the ToolShell SharePoint zero-day attacks, including impact, victims, and threat actors.